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The Angle of Attack of a rotor blade is the angle between the chord line and the...
  • A
    Plane of rotation.
  • B
    Relative Airflow.
  • C
    Rotational Airflow.
  • D
    Iinduced Airflow.

Refer to figure.
Angle of Attack (AoA, α or alpha) is the angle between the Relative Airflow and the Chord line.

AoA constitutes one of the elements of the Coefficient of Lift (CL) and affects its value. More specifically, Lift increases (up to a point) as AoA increases.

The point of maximum lift is called "Critical AoA" or "Critical alpha" and the relationship is roughly linear up to 15º approximately for a symmetric airfoil.

Beyond this point, the amount of lift produced decreases as the AoA increases further. The AoA beyond the point which lift starts to decrease is known as "Stalling AoA".

The Plane of Rotation, also known as the Hub Plane of Rotation is parallel to the tip path plane but runs through the centre of the hub.
Induced Airflow is the mass of the air forced downwards by the rotor blades and is represented by a vertical vector.
The Rotational airflow is due to the rotation of the rotor which varies from root to tip. It is the component of flow caused by a combination of the rotational speed of the blade (NR) plus the component of flow due to the horizontal speed of the helicopter. If added to the Induced Airflow, it gives the effective Relative Airflow.

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